Large, bright light streaking smooth and low across the night sky, off the side of the highway.I was driving south on I84, between Twin Falls and Ogden, somewhere between 11:30 on Aug 20th and 0:30 on Aug 21. There was one other car in the vicinity traveling in the same direction. That car had 2 occupants (male and female, I believe). Off to the right ride of the Interstate, there were some hills a few hundred meters away. A bright white-yellow light became apparent over the hills in my peripheral vision. I turned to watch the light, moving much faster than my car (70mph) and at least 200 meters away. It was traveling in a mostly S direction (the same as my car), streaking down in a 30-40 degree angle, and fast approaching the hills. At first I thought it was a crashing plane, but the speed and the angle of the motion resembled no plane that I have ever seen. The trajectory was completely smooth. The size of the light was far too big to be a shooting star. As it approached the hills, the light's speed slowed dramatically. Then the light disappeared completely, perhaps falling below the hills. After the light disappeared, I slowed my car down considerably to study the hills. There was no smoke, no sound, nothing. The other car traveling near me slowed down as well. We were both going between 30 and 40 mph. I looked into the front of the other car and the two occupants were both looking towards the hills as well. I felt that they had seen the light as well and were also trying to make sense of it. (The reason I am posting this now is that I just read about this site).((NUFORC Note: Witness indicates that the date of the incident is approximate. PD))